Sulfur trioxide is generally commercially produced from sulfur-containing substances such as free sulfur, hydrogen sulfide, sludge acid, pyrites, and other materials, by combusting these in the presence of air or oxygen to produce a gas containing predominantly sulfur dioxide. The sulfur dioxide is then subjected to several stages of catalytic conversion, usually utilizing a platinum or vanadium catalyst, to produce a gas containing sulfur trioxide. Sulfur trioxide is then recovered either by absorption with sulfuric acid to produce oleum or by absorption of a portion of the sulfur trioxide with sulfuric acid, and condensing the remainder of the sulfur trioxide in a cooled condensation apparatus to produce sulfur trioxide in liquid form. It has also been found that the production of sulfur trioxide is enhanced by conducting the process at elevated pressures, generally 3 atmospheres and above.
For example, U.S. Pat. No. 3,803,297 to Guth et al. discloses the production of sulfur trioxide by combustion of sulfur with oxygen in at least two stages, each of which is preferably subdivided into sub-stages, followed by the cooling of the combustion gases, catalytic conversion of the gases to sulfur trioxide and recovery of sulfur trioxide by absorption with sulfuric acid to produce oleum. The temperature in the combustion stages is controlled by (a) utilizing less than a stoichiometric amount of oxygen and (b) passing some of the feed oxygen through the sulfuric acid absorption stage to pick up some sulfur trioxide, thereby recycling this sulfur trioxide to the combustion stage to act as a heat sink (the decomposition of sulfur trioxide is an endothermic reaction). The patent requires a careful control of combustion by dividing it into two main stages, with from about 5-40% of the sulfur being combusted in the first stage together with at most a stoichiometric amount of oxygen, and recycled sulfur trioxide, and the remaining 50-95% sulfur being oxidized in a second stage using a stoichiometric amount of oxygen. The second stage is additionally divided into a series of sub-stages for better control of the operating temperature. Some sulfur trioxide is also introduced into the gases entering the sulfur dioxide catalytic conversion steps.
Another such process is disclosed in U.S. Pat. No. 3,455,652 of James. A sulfur-containing feed is combusted with air or oxygen-enriched air, the oxygen being present in excess, the sulfur dioxide being converted to sulfur trioxide in a conventional catalytic conversion step or steps, and the sulfur trioxide being recovered by condensation of one portion and absorption of the remaining portion with sulfuric acid to produce oleum. The process is conducted at elevated pressures, between 3 and 50 kg/cm.sup.2.
It is known, for example, from U.S. Pat. No. 3,803,297, that the combustion of sulfur with stoichiometric amounts of pure oxygen results in extremely high temperatures in the combustion furnace (about 3000.degree. C. absent the use of cooling means). This temperature can be lowered to below 1200.degree. C., as in U.S. Pat. No. 3,455,652, by conducting the combustion with air or oxygen-enriched air. The drop in temperature, however, is more than offset by the dilution of process gases with large volumes of inert gases, particularly nitrogen, which requires large equipment and higher utilities for processing the overall gas streams, and large-scale constant venting with pollutant removal to prevent buildup of inerts in the system.
It is an object of the present invention to provide an improved process for the production of liquid sulfur trioxide.
It is a further object of the present invention to provide an improved process for the production of liquid sulfur trioxide at elevated pressures.
It is a further object of the present invention to provide a process for the production of liquid sulfur trioxide with relatively low temperatures in the sulfur combustion zone.
It is another object of the present invention to provide a process for the production of liquid sulfur trioxide without requiring the necessity of elaborate equipment or processing steps in the combustion zone.
It is also an object of the present invention to provide a process for the production of liquid sulfur trioxide having a combustion stage which operates at a low temperature and with conventional equipment, without the necessity of recycling large quantities of inert gases.
Another object of the present invention is to provide such a process which permits the use of elemental oxygen as the combustion gas.
Still another object of the present invention is to provide a process for the production of sulfur trioxide which facilitates recovery of the desired product.
Another object of the present invention is to provide a process for the production of liquid sulfur trioxide without the concomitant production of oleum or sulfuric acid.
Yet another object of this invention is to provide a process for the production of liquid sulfur trioxide in which potential pollution sources are minimized.